This invention relates to ultrasonic inspection, and particularly to an ultrasonic inspection apparatus utilizing a laser to generate acoustic waves at ultrasonic frequencies.
The invention has many uses, including, for example, monitoring the structural integrity of components of aircraft, ships, submarines, land vehicles, and pressure vessels. Parameters that can be monitored include, but are not limited to, strain, temperature, crack initiation, corrosion, impact damage, delamination and bond integrity.
Many advances have been made in laser ultrasonics over the past decade. A typical laser ultrasonic system utilizes two lasers, one for ultrasound generation, and the other for ultrasound detection.
The first laser is typically a high power (0.1-1.0 Joule) pulsed laser producing 3 nanosecond pulses. The wavelength and laser pulse energy must be chosen so that the energy density produced by the laser in the material being inspected is below the ablation threshold of the material being inspected (typically about 0.5-1.0 J/cm.sup.2). However, the wavelength must be such that the laser energy is absorbed inside the material and not just at the surface.
The second laser is typically a continuous wave (cw) laser which operates collinearly with the first laser. Light from the second laser, reflected from the surface of the material being inspected, is transmitted to a device for detecting Doppler shift in the reflected light beam, such as a Michelson interferometer, a heterodyne interferometer or a confocal Fabry-Perot interferometer.
The pulsed laser produces ultrasonic vibrations within the material, and at the surface which reflects light from the second laser. These vibrations produce a Doppler shift in the reflected light which is proportional to the surface displacement. Defects within the material produce changes in the magnitude, frequency and/or damping of the vibrations, which can be detected by observing the Doppler shift using an interferometer.
Laser ultrasonic inspection has advantages over conventional immersion tank ultrasonic systems. It does not require direct contact with the material being inspected, and can be used for remote scanning. It also operates well on materials having curved surfaces and can be easily used to scan large areas. However, laser ultrasonic inspection also has disadvantages. Unlike an imaging or wide-area method, laser ultrasonic inspection is essentially a point inspection method, in which only one point on the surface of a material can be inspected at a given instant of time. Another disadvantage is the potential for the laser to cause damage to the surface of the material being inspected. Still another disadvantage is that the high power laser beam travels through open air, and therefore requires expensive safety systems. Conventional laser-ultrasonic inspection systems are also very expensive and large in size. In general, it is not practical to incorporate them into a structure such as an aircraft wing, for periodic checking of the integrity of the structure.